The present invention relates to a method for controlling the transmission power in a radio system; in particular, in a mobile radio system.
Continuous control is provided for the transmission power of the base stations and the mobile stations in mobile radio systems such as those according to the UMTS mobile radio standard (“Universal Mobile Telecommunication System”). In this case, the transmission power is controlled, in particular, in the form of a closed control loop (closed loop power control). For this purpose, a transmitter (for example, a mobile station), transmits a specific transmission signal (which is also referred to as a pilot signal) to the receiver (for example, a base station). The pilot signal in this case includes one or more pilot bits which are evaluated by the base station in order, as a function of it or them, to determine the signal-to-interference ratio (SIR) which is received on the respective transmission channel, and which is compared with a nominal value. The base station then transmits a power control command to the mobile station, whose aim is to increase or reduce the transmission power of the mobile station depending on the result of the comparison.
In order to illustrate this process, FIG. 3 shows the communication between a mobile station MS and a base station BS in a mobile radio system; for example, in a UMTS mobile radio system. In the following text, it is assumed that the aim is to control the transmission power of the mobile station MS. The mobile station MS transmits the already-described pilot signal via the transmission channel from the mobile station to the base station, which is referred to as the “uplink” transmission channel, and this pilot signal is evaluated by the base station BS. The base station BS then produces power control commands, which are transmitted to the mobile station MS via the transmission channel from the base station to the mobile station, which is referred to as the downlink transmission channel. Generally, these power control commands relate only to information instructing the mobile station (MS) to increase or to reduce its transmission power by a predetermined amount, or else to leave it unchanged. In principle, the power control command may include only one bit which, depending on its value, instructs the mobile station MS to increase or to reduce its transmission power by the abovementioned value.
The communication between the mobile station MS and the base station BS takes place in the form of a frame and time slot structure; in particular, with a new pilot signal being transmitted by the mobile station MS in each time slot. In a corresponding way, the base station BS transmits a new power control command in each time slot to the mobile station MS, with this power control command being based on the pilot signal transmitted during the preceding time slot.
A so-called compression mode is defined for the UMTS mobile radio standard, which envisages the information to be transmitted via the uplink and/or downlink transmission channel being transmitted in compressed form. This means that a compressed frame, transmitted in the compressed mode, includes one or more time slots in which no information is transmitted. These time slots form a transmission gap which can be used, inter alia, for adjacent channel observations, to prepare for handover processes, or the like.
When a transmission gap such as this occurs in the uplink and/or downlink channel, the base station BS does not receive the pilot signal (which is required to produce the power control commands) from the mobile station MS, or the mobile station MS does not receive the corresponding power control command from the base station BS. A transmission gap such as this in the uplink and/or downlink transmission channel thus interferes with the closed control loop which is used to control the transmission power of the mobile station MS, so that no power control commands can be transmitted during such a transmission gap.
The above description is also applicable, of course, to the control of the transmission power of the base station BS, although the present invention will be described in the following text—(without any restriction to generality) on the basis of the control of the transmission power of the mobile station MS.
In order to solve the transmission power control problem previously associated with the occurrence of a transmission gap, proposals have been made with regard to the UMTS mobile radio standard for an estimated value for the power control command to be expected in the future to be produced on the basis of the power control commands produced in the past, so that this estimated value can be used as the basis for controlling the transmission power after the occurrence of a transmission gap. In this case, the estimated value δi, which describes the transmission power change to be expected after a transmission gap, can be determined, in particular, with the aid of the following recursive equations, with TPCi denoting the power control command received in the time slot No. i, and ΔTPC denoting the step width which is available for adjusting the transmission power. δi−1 denotes the estimated value determined for the preceding time slot i−1:δi=0.9375·δi−1−0.96875·TPCi·ΔTPC  (1)δi−1=δi  (2)
When controlling the transmission power of a mobile station (see FIG. 3), the estimated value δi is intended to be recalculated continuously by the mobile station MS in all the time slots in which both an uplink pilot signal and a downlink power control command are transmitted. Furthermore, the estimated value δi is also calculated in the first time slot of an uplink transmission gap if a downlink power control command has been transmitted in the corresponding time slot. The value δi−1 is set to 0 whenever the control channel associated with the uplink channel of the mobile station MS is initiated or activated. Furthermore, δi−1 is reset to 0 at the end of the first time slot after each uplink transmission gap and at the end of the first time slot after each downlink transmission gap. The estimated value for δi is, in each case, set to 0 at the end of the first time slot after each uplink transmission gap.
However, one problem that is associated with the above formulae and with the procedure described above occurs when the transmission power emitted by the mobile station MS is close to a predetermined maximum transmission power Pmax or to a minimum transmission power Pmin. Proposals have been made for UMTS mobile radio systems to carry out a scaling process in the vicinity of such transmission power limit values, in order to prevent the predetermined minimum transmission power Pmin being significantly undershot, and/or the predetermined maximum transmission power Pmax being significantly exceeded. This scaling process results in the above formula (1) being replaced by the following formula:δi=0.9375·δi−1−0.96875·TPCi·ΔTPC·k  (3)
In this case, k denotes a parameter which assumes the value “0” when scaling is intended to be carried out, while the parameter k has the value “1” when no scaling is intended to be applied and the transmission power is intended to be determined in the normal manner.
The parameter k is conventionally set to k=0 whenever the transmission power emitted by the mobile station MS on the basis of previous power control commands would exceed the predetermined maximum transmission power Pmax or would be less than the predetermined minimum transmission power Pmin. In other situations, k=1 is used.
The scaling process to be carried out by the mobile station MS can be chosen such that power control commands which cause a further reduction in the transmission power of the mobile station are, in principle, multiplied by a specific factor, such as ¼, and are attenuated in the vicinity of the minimum transmission power Pmin, while power control commands which cause an increase in the transmission power are used without scaling.
FIG. 5A shows a corresponding profile of the transmission power. As can be seen from FIG. 5A, the mobile station MS is already close to the minimum transmission power limit Pmin. The mobile station then receives a power control command which reduces the transmission power, and which would result in the minimum transmission power limit Pmin being undershot. The mobile station MS applies the previously mentioned scaling process and multiplies each subsequent power control command which would cause a reduction in the transmission power by the factor ¼, so that these power control commands are attenuated. Since the scaling process is used for these power control commands, the parameter k has the value k=0. At the end of the transmission power profile shown in FIG. 5A, the mobile station MS receives a power control command which causes the transmission power to be increased. This power control command is implemented without any scaling, so that the parameter k=1 is used for this power control command. If the transmission power profile explained above is considered from the point of view of the already-described δ formula, then those power control commands which would lead to a transmission power below the minimum transmission power limit Pmin would not be included in the tendency of the power control commands in the δ formula by setting the parameter k to “0”, while power control commands which result in a sudden change in the transmission power upward and beyond the minimum transmission power limit Pmin would be included in the estimated value δ. If the transmission power profile shown in FIG. 5A were repeated precisely in this manner several times successively, then a considerable increase in the transmission power would be taken into account as the tendency of the transmission power commands in the δ formula, although this does not correspond to reality.
When using the already-described scaling method, the mobile station MS is free to choose whether it will or will not allow the minimum transmission power limit Pmin to be undershot. If the mobile station MS is not intended to support undershooting of the minimum transmission power limit Pmin, then this can result in a transmission power profile as shown in FIG. 5B when the conventional scaling method is used. In principle, in the example shown in FIG. 5B, the scaling process is activated on reaching the minimum transmission power limit Pmin in such a way that transmission takes place with the value of the minimum transmission power limit Pmin. The parameter is set to k=0 once again on reaching the minimum transmission power limit Pmin. Otherwise, the principle already explained with reference to FIG. 5A still applies.
A similar problem also can occur at the maximum transmission power limit Pmax, in which case the scaling process may be chosen, for example, such that, when a power control command occurs which would lead to the maximum transmission power limit Pmax being exceeded, this power control command is reduced such that the transmission is carried out only at the magnitude of the maximum transmission power Pmax. When a power control command occurs which would lead to undershooting of the maximum transmission power limit, no scaling is used, in contrast.
FIG. 5C shows a corresponding transmission power profile. As can be seen from FIG. 5C, a power control command which would lead to the maximum transmission power limit Pmax being exceeded is subjected to the scaling process; that is to say, the parameter k is set to k=0, and the power control command is reduced such that transmission is carried out only at the value of the maximum transmission power Pmax. If, on the other hand, a power control command occurs which reduces the transmission power of the mobile station MS, no scaling is used (k=1) and this power control command is fully implemented, so that the maximum transmission power limit is considerably undershot.
Analogously to the examples illustrated in terms of the minimum transmission power limit Pmin in FIGS. 5A and 5B, the problem also occurs in the case of the example illustrated with regard to the maximum transmission power limit Pmax in FIG. 5C that, if repeated a number of times, would result in a power control response as a tendency which does not correspond to reality. In particular, in the example shown in FIG. 5C, a reduction in the transmission power which does not correspond to reality would be taken into account as the tendency of the power control commands.
The present invention is, therefore, directed toward an improved method for controlling the transmission power in a radio system, in particular a mobile radio system, wherein the problems mentioned above can be avoided and exact and realistic control of the transmission power is possible even in the area of the transmission power limits.